1. Field
The present disclosure relates to a fixing device used for a copier, a printer, a facsimile, and a multifunctional peripheral thereof, and to an image forming apparatus including the fixing device. In particular, the present disclosure relates to an electromagnetic induction heating type fixing device and an image forming apparatus including the fixing device.
2. Background
Conventionally, the electromagnetic induction heating type fixing device has a structure in which a magnetic flux generated by an exciting coil causes eddy current in an induction heating layer disposed in a heating member, and the induction heating layer is heated by Joule heat generated by the eddy current so that the heating member is heated to a predetermined fixing temperature. This type of fixing device can reduce thermal capacity of the induction heating layer. Therefore, a warm-up time for starting the device can be shortened, and hence compact and high thermal conversion efficiency can be obtained. However, in the case of small size of paper sheet to be conveyed, a paper passing region of the heating member through which the paper sheet passes is cooled to be low temperature as the paper sheet absorbs heat from a surface of the heating member, while a non-paper passing region through which the paper sheet does not pass remains at high temperature. In particular, when the paper passing region of the heating member is maintained at fixing temperature in the case where paper sheets pass through continuously, temperature of the non-paper passing region of the heating member rises excessively so that temperature of the heating member or the exciting coil exceeds its heat resistance limit temperature resulting in a malfunction such as a breakdown of the member.
Therefore, related techniques for solving the above-mentioned malfunction are proposed. Fixing devices of a first related technique and a second related technique include a magnetic core having Curie temperature set to be higher than the fixing temperature, and a coil which generates a magnetic flux for heating the heating member by electromagnetic induction with the magnetic core. Further, the magnetic core has different Curie temperatures in a direction perpendicular to a paper conveying direction, in order to prevent a large difference of temperature between the paper passing region with which the paper sheet contacts and the non-paper passing region with which the paper sheet does not contact on the surface of the heating member, in the case where a lot of small size of paper sheets are fixed continuously. In other words, end magnetic cores on both end portions corresponding to the non-paper passing region have Curie temperature lower than that of a middle magnetic core corresponding to the paper passing region. Therefore, in the case where a toner image is fixed on a small size of paper sheet, when the temperature of the non-paper passing region rises excessively so that the temperature of the end magnetic core becomes the Curie temperature or higher by heat radiation or heat conduction from the heating member, a heating value of the non-paper passing region is decreased because of a decrease of magnetic permeability of the end magnetic cores. As a result, the temperature of the non-paper passing region of the heating member is lowered.
In addition, a fixing device of a third related technique includes a plurality of magnetic cores arranged in the direction perpendicular to the paper conveying direction along the coil which generates the magnetic flux for induction heating of the heating member. The Curie temperature of a portion of the magnetic core corresponding to the non-paper passing region is set to a temperature range between the temperature of the portion of the magnetic core corresponding to the non-paper passing region when the temperature of the heating member becomes the fixing temperature or higher and the temperature lower than the temperature of the portion of the magnetic core corresponding to the non-paper passing region when the temperature of the heating member or the coil becomes the heat resistance temperature. Thus, the paper sheet can be heated and fixed while preventing the temperatures of the heating member and the coil from exceeding the heat resistance limit temperature resulting in a breakdown of the member.
In addition, in a fixing device of a fourth related technique, the magnetic core includes a plurality of trapezoidal first magnetic cores arranged in the direction perpendicular to the paper conveying direction so as to cover the coil which generates a magnetic flux for induction heating, and a plurality of second magnetic cores arranged in the direction perpendicular to the paper conveying direction in a gap formed by a loop of a coil wound in a looped shape. The Curie temperature of the end magnetic cores disposed corresponding to the non-paper passing region among the second magnetic cores is set lower than the Curie temperature of the first magnetic core. Further, because the end magnetic cores are disposed separately from the first magnetic core, the thermal capacity thereof is smaller than that of the first magnetic core. Therefore, when the temperature of the non-paper passing region rises excessively, the temperature of the end magnetic cores rise relatively fast by thermal radiation or heat conduction from the heating member to the end magnetic cores so that excessive rise of temperature of the heating member in the non-paper passing region is prevented rapidly.
Usually, in order to rapidly suppress excessive rise of temperature of the heating member in the non-paper passing region, it is necessary to set good temperature following property of the end magnetic cores to a temperature variation of the heating member. In addition, for example, the coil is wound in a loop shape a plurality of turns, and is formed and cured by heating to melt a melting layer on the surface of the coil. Therefore, it is necessary to cool the coil so that the coil temperature does not rise to a predetermined temperature (coil cooling temperature) or higher in order to prevent the coil temperature from rising excessively so that the coil is broken or a shape of the coil is lost. Affected by cooling the coil, the temperature of the end magnetic cores disposed close to the coil is not raised to the coil cooling temperature or higher. Therefore, the Curie temperature of the end magnetic cores should be set in view of the above-mentioned discussion. In other words, it is necessary to set the Curie temperature of the end magnetic cores disposed close to the coil to the temperature in view of the temperature cooling the coil. However, in the fixing devices of the above-mentioned first to third related techniques, the end magnetic cores do not have good temperature following property to a temperature variation of the heating member. In addition, in the fixing device of the fourth related technique, the Curie temperature of a lower limit of the end magnetic cores is set, but the Curie temperature of an upper limit is not set. Therefore, the temperature of the heating member may exceed the heat resistance limit temperature so that the heating member may be broken.